What the two rings mean
The inner ring shows ten natal planets at the recorded birth instant. The outer ring recalculates those ten planets at the selected return instant. The center is a fixed tropical zodiac, beginning with 0° Aries at the left. Short ticks mark natal and return house cusps in their separate rings. Planetary positions and house assignments are also listed in text so close placements remain readable.
A return is an exact longitude match. The calculator takes the selected planet’s apparent geocentric tropical longitude at birth, searches forward from the requested date, and refines the next crossing of that same longitude. The rest of the sky does not repeat. A Mercury return chart, for example, contains Mercury at its natal degree while the Moon, Venus, angles, and houses describe the later return moment.
Solar, lunar, and planetary searches
A solar return search checks a sixteen-day window around the local birth anniversary in the selected year. A lunar return starts at 00:00 UTC on the chosen date and finds the next Moon crossing, normally within one sidereal month. Mercury, Venus, Mars, Jupiter, and Saturn use bounded planet-specific searches: two years for Mercury, three for Venus, five for Mars, fifteen for Jupiter, and thirty-three for Saturn, ending earlier at the year-2100 calculation limit.
The planetary search samples longitude often enough for the selected body, then refines every true crossing and rounds the result to the nearest UTC minute. The displayed residual states the small longitude difference after that rounding. A retrograde loop can carry the planet across its natal degree three times. When that happens, the result lists every direct and retrograde crossing and lets you redraw the biwheel for any one of them instead of silently choosing a preferred event.
Worked example: three Saturn crossings
Suppose Saturn first reaches its natal longitude while moving direct, reverses across the same degree several months later, and returns a third time after stationing direct. All three moments satisfy the return equation. Select each row to compare the surrounding sky. The natal ring remains fixed; the return planets, angles, and houses change with the chosen crossing. The motion label describes Saturn’s direction at the exact match and does not rank one crossing as more important.
This separation is useful when a long return period creates a cluster rather than a single date. The event selector preserves the measured sequence, while the chart sheet shows the different context around each crossing. The print action uses the currently selected event, so the event time and motion label remain attached to the wheel and tables.
Two places, two sets of houses
The birth coordinates set natal Ascendant, Midheaven, and house cusps. The return coordinates set a second local horizon at the worldwide return instant. Moving the return place changes the return angles and houses, not the UTC return moment or geocentric planetary longitudes. The same requested house system is applied to both charts, and the result names the effective system separately for each.
Whole Sign places each sign on a house; Equal House starts each thirty-degree house at the Ascendant. Porphyry divides each quadrant between the Ascendant and Midheaven into three equal ecliptic arcs. Placidus uses temporal divisions and has geographic limits. When Placidus is unavailable, the engine labels a Porphyry fallback. At the geographic poles, angles and houses can be undefined and are withheld rather than invented.
Birth time and time zones
A precise return target requires the birth instant. The Moon can travel more than a zodiac degree in a few hours, and even a slower planet’s exact target depends on the recorded time. This form withholds every exact-return mode when the time is unknown. Enter the civil UTC offset that applied at the birthplace on the recorded date, including daylight-saving and historical changes. The birthplace atlas can help check a place and time zone.
The event date is reported in UTC. The entered return coordinates set the chart’s local horizon but do not infer the event’s civil clock time or daylight-saving rule. Convert the UTC instant using the rules that applied at the chosen return place if you need a local timestamp. Do not reuse the historical birth offset automatically.
Ephemeris model and calculation limits
- Astronomy Engine 2.1.19 documents the locally bundled MIT-licensed position model used for geocentric vectors.
- NASA/JPL Horizons documentation describes an independent ephemeris service that can be used to compare positions and UTC conventions.
- Planetary return dates exposes every Mercury-through-Saturn crossing and a downloadable CSV before a chart location is chosen.
These outputs are astronomical coordinates and traditional chart constructions. The calculator does not generate event predictions or claim that one crossing causes an outcome.